Uninline epoch(9) entrance and exit. There is no proof that modern
processors would benefit from avoiding a function call, but bloating
code. In fact, clang created an uninlined real function for many
object files in the network stack.

• Move epoch_private.h into subr_epoch.c. Code copied exactly, avoiding any changes, including style(9).
• Remove private copies of critical_enter/exit.

Reviewed by: kib, jtl
Differential Revision: https://reviews.freebsd.org/D17879